Module Path Substitution

**Is your feature request related to a problem? Please describe.**
Yes. When hosting bicep modules in a registry would like to either a.) substitute the registry URL per environment or b.) substitute the tag in the module location for the specific environment/version.

This is an issue with teams who may have different module definitions for dev/test/prod or have the desire to run things like a beta module in their dev environment. Currently the module reference requires a path that cannot be interpopulated. Other IaC providers get around this with the `source` property in the module block.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.

Something like:
module appService '${parameters.registryURL}:${parameters.registryTag}'